1. Field of the Invention
The present invention relates to an optical transmission system for Raman amplifying signal light to transmit the amplified signal light, in particular, to an optical transmission system for Raman amplifying signal light by supplying pumping light generated by a Raman amplifier common to both an upstream line and a downstream line.
2. Description of the Related Art
With conventional long distance light transmission systems, optical transmission has been performed using optical regeneration repeaters that convert optical signals into electric signals to effect re-timing, re-shaping, and re-generating. However recently, with progress in the utilization of optical amplifiers, optical amplifying-and-repeating transmission systems that use optical amplifiers as linear repeaters are being investigated. By replacing an optical regeneration repeater with an optical amplification repeater, the number of parts in the repeater can be greatly reduced, with the expectation of maintaining reliability and greatly reducing costs. Furthermore, as one method of realizing a large capacity of an optical transmission system, a wavelength division multiplexing (WDM) optical transmission system that multiplexes two or more optical signals having different wavelengths to transmit the multiplexed light on a single optical transmission path is attracting attention.
In a WDM optical amplifying-and-repeating transmission system configured by combining a WDM optical transmission system with an optical amplifying-and-repeating transmission system, it is possible to collectively amplify two or more optical signals having different wavelengths using an optical amplifier, thus enabling the realization of large capacity and long distance transmission with a simple (economic) construction.
FIG. 11 is a diagram showing an exemplary configuration of a conventional WDM optical amplifying-and-repeating transmission system.
The system in FIG. 11 comprises, for example, an optical sender station 101, an optical receiver station 102, an optical transmission path 103 connecting between the sender station and the receiver station, and a plurality of optical repeater stations arranged along the optical transmission path 103 at required intervals. The optical sender station 101 has a plurality of optical senders (E/O) 101A that output a plurality of optical signals having different wavelengths respectively, a multiplexer 101B that wavelength multiplexes the plurality of optical signals, and a postamplifier 101C that amplifies WDM signal light from the multiplexer 101B to a required level and outputs it to the optical transmission path 103. The optical receiver 102 includes a preamplifier 102C that amplifies the WDM signal light of each wavelength band transmitted via the optical transmission path 103 to a required level, a demultiplexer 102B that demultiplexes the output light from the preamplifier 102C into a plurality of optical signals according to the wavelengths, and a plurality of optical receivers (O/E) 102A that receive the plurality of optical signals, respectively. The optical transmission path 103 includes a plurality of transmission sections that connect between the optical sender station 101 and the optical receiver station 102. The WDM signal light sent from the optical sender 101 is propagated through the optical transmission path 103, optically amplified at the optical repeater station 104 arranged at each transmission section, and then propagated through the optical transmission path 103 again, and thereafter, the WDM signal light is transmitted to the optical receiver section 102 by repeating the above steps.
As the optical repeater station 104 in the WDM optical amplifying-and-repeating transmission system described above, for example, an erbium doped optical fiber amplifier (EDFA) is typically used. Also, the use of EDFA in combination with Raman amplification is extensively considered recently. Further, a repeaterless optical transmission system that does not use the optical repeater station is proposed, wherein remote-pumping that controls distributed Raman amplification and the like is considered.
In the Raman amplification using an optical fiber as an amplification medium, a gain to be obtained is in inverse proportion to a mode field diameter of the used optical fiber. Therefore, an optical fiber having a smaller mode field diameter is suitable for Raman amplification. For example, a negative dispersion fiber having wavelength dispersion and a dispersion slope (first-order differential of wavelength dispersion with respect to the wavelength) of sign opposite to wavelength dispersion and a dispersion slope of 1.3 μm zero dispersion fiber, is of the mode field diameter of about 5 μm smaller than the mode field diameter of the 1.3 μm zero dispersion fiber or a dispersion-shifted fiber (DSF, NZ-DSF) typically used as the optical transmission path. Therefore, larger Raman gain can be obtained.
Here, the positive dispersion fiber such as the 1.3 μm zero dispersion fiber or the dispersion-shifted fiber is abbreviated as +D fiber, and the negative dispersion fiber as mentioned above is abbreviated as −D fiber in the following description.
Further, in the conventional WDM optical repeating transmission system, a method for managing the wavelength dispersion of the optical transmission path has been used in order to reduce degradation of transmission characteristics of the optical transmission path due to a nonlinear effect. For example, in article 1; “Long-haul 16×10 WDM transmission experiment using higher order fiber dispersion management technique”, M. Murakami et al., pp. 313–314, ECOC'98, 1998, there is proposed a technique for compensating for, in one transmission section (a compensation section) using the +D fiber, cumulative dispersion occurred in a plurality of transmission sections using a hybrid transmission path in which the +D fiber and the −D fiber are combined. More specifically, an average zero-dispersion wavelength of the optical transmission path shown in the article 1 is about 1551 nm, and a signal light wavelength is 1544.5 nm–556.5 nm. Further, the respective wavelength dispersion of each transmission section using the hybrid transmission path and the compensation section using the +D fiber are about −2 ps/nm/km and about +20 ps/nm/km, respectively. According to such configuration, since a group speed between signal light and spontaneous emission light and a group speed among a plurality of signal light are different from each other, an interaction time by the nonlinear effect can be shortened, thereby enabling to reduce the degradation of the transmission characteristics due to four wave mixing (FWM), cross phase modulation (XPM) and the like. Further, since the average zero-dispersion wavelength is kept within signal light wavelength, the degradation of the transmission characteristics due to self wave modulation (SPM) and the wavelength dispersion can also be reduced.
When a distributed Raman amplifier is applied to the conventional WDM optical repeating transmission system described above, it is difficult to obtain a Raman gain efficiently by using the +D fiber, since the +D fiber has a larger mode field diameter than the −D fiber. As a result, there is caused a problem in that significantly large pumping light power is needed to obtain the Raman gain required to compensate for losses in sections using the +D fiber, causing a disadvantage in terms of reliability of pumping light source and the like. To overcome the above problem, for example, it is contemplated to apply a Raman amplification fiber having a smaller mode field diameter and shorter length than the −D fiber so that the Raman gain can be obtained more efficiently to compensate for the losses in the sections of the +D fiber.
However, when the fiber for Raman amplification having the smaller mode field diameter as described above is used, there is caused a problem in that the nonlinear effect occurring in the fiber for Raman amplification on signal light may be increased. Further, there is caused a disadvantage in that many types of optical amplifiers must be used since configurations for realizing the distributed Raman amplification in the −D fiber and for realizing concentrated Raman amplification in the fiber for Raman amplification are needed. Still further, there may be a problem in that distortion of a transmission waveform may be increased due to the nonlinear effect in the entire optical transmission system.
As another method for managing the wavelength dispersion by using the hybrid transmission path configured by the combination of the +D fiber and the −D fiber, there is proposed, as shown, for example, in article 2; “1800 Gb/s transmission of one hundred and eighty 10 Gb/s WDM channels over 7,000 km using the full EDFA C-band”, C. R. Davidson et al., PD25, OFC2000, 2000, and the like, a method in which the cumulative wavelength dispersion per one section of the hybrid transmission path is reduced to substantially zero and the cumulative dispersion that may occur during transmission is compensated at a terminal station.
However, when the cumulative wavelength dispersion per one section of the hybrid transmission path is reduced to substantially zero, wavelength degradation due to SPM can be alleviated, but, on the other hand, wavelength degradation due to XPM may cause a problem, since a state in which the bit arrangements among wavelengths become the same in regions experiencing the same amount of the nonlinear effect may occur in each transmission section.
In consideration of the above problems, the inventors of the present application have proposed a technique for compensating for wavelength dispersion by configuring a optical transmission path by combining a hybrid transmission path in which positive cumulative wavelength dispersion is caused with a hybrid transmission path in which negative cumulative wavelength dispersion is caused, in the optical transmission system (Japanese Patent Application 2001-075721).
FIG. 12 is a diagram showing an exemplary configuration of the optical transmission system according to the prior application mentioned above. In this system configuration, same pumping light sources are used for both upstream and downstream lines at each optical repeater station and a unitary system set up of the upstream and downstream lines is pumped by one Raman amplifier to perform Raman amplification. According to such a configuration, since pumping light is incident to the −D fiber in all transmission sections, it is possible to obtain the Raman gain efficiently and also to reduce the types of optical amplifier to one type.
However, in the optical transmission system as shown in FIG. 12, when the distributed Raman amplification is performed in each transmission section, in the upstream and downstream lines, there appears portions where two types of the transmission sections in which average wavelength dispersion is opposite to each other in positive/negative sign are pumped by the common Raman amplifier, as shown in FIG. 13, for example. Therefore, there is caused a disadvantage in that it is difficult to control the Raman gain in each line
Namely, in order to adjust the wavelength dispersion, the −D fiber of each transmission section in which average wavelength dispersion is positive abbreviated as the sign “+” in FIG. 13, is set to be different in length from the −D fiber of each transmission section in which average wavelength dispersion is negative abbreviated as the sign “−” in FIG. 13. As a consequence, if one common Raman amplifier is used for both the upstream and downstream lines in each repeater as shown in FIG. 12, there appears portions where two types of the transmission sections in which the average wavelength dispersion is opposite to each other in positive/negative sign are pumped by the common Raman amplifier, as shown in FIG. 13, as shown in the parts enclosed by dotted lines in FIG. 13.
FIG. 14 is an illustrative diagram showing the parts enclosed by dotted lines in FIG. 13 in an enlarged manner. Here, pumping light output from a pumping light source 200 is branched into two by an optical coupler 201. One branched light is supplied via a multiplexer 202A from the side of a −D fiber 203B to a transmission section of the upstream line, in which the lengths of a +D fiber 203A and the −D fiber 203B are adjusted so that average wavelength dispersion has negative sign. The other branched light is supplied via a multiplexer 202B from the side of the −D fiber 203B to the other transmission section of the downstream line, in which the lengths of the +D fiber 203A and the −D fiber 203B are adjusted so that average wavelength dispersion for the section has positive sign. At this time, the Raman gain caused in each transmission section of the upstream and downstream lines, differs significantly between the upstream line side and the downstream line side, since an absolute value thereof is changed according to the length of the −D fiber 203B.
As a specific example, in order to set average wavelength dispersion to −2.7 ps/nm/km for a transmission section of 50 km, the lengths of the +D fiber 203A and the −D fiber 203B may be set to 32.5 km and 17.5 km, respectively. On the other hand, in order to set the average wavelength dispersion to +2.7 ps/nm/km for the transmission section of 50 km, the lengths of the +D fiber 203A and the −D fiber 203B may be set to 36.7 km and 13.3 km, respectively. Here, assuming that the transmission section of the upstream line, in which the average wavelength dispersion is set to −2.7 ps/nm/km, and the transmission section of the downstream line, in which the average wavelength dispersion is set to +2.7 ps/nm/km, are pumped by the common Raman amplifier, a difference of the Raman gain between the upstream line and the downstream line is about 0.5 dB according to calculation using parameters shown in Table 1 below.
TABLE 1+D/−D fibers+D/−D fibersFiber(upstream line)(downstream line)Average wavelength dispersion−2.7+2.7(ps/nm/km)Length (km)32/1836/14Nonlinear effective cross-sectional110/18 ←area (μm2)@ signal light wavelengthNonlinear effective cross-sectional106/15 ←area (μm2)@ pumping light wavelengthTransmission losses (dB/km)0.18/0.28←@ signal light wavelengthTransmission losses (dB/km)0.22/0.55←@ pumping light wavelengthNonlinear refractive index2.8/4  ←coefficient (x10−20 m2/W)Raman gain coefficient (x10−14 m/W)1.7/3.4←
Further, in the optical transmission system in which the distributed Raman amplification is performed by pumping each transmission section of the upstream and downstream lines using the common Raman amplifier, there is caused a disadvantage in that it is difficult to control the Raman gain when any failure occurs and the like, irrespective of whether the hybrid transmission path is applied or not.
Namely, for example, as shown in FIG. 15, assuming that a failure occurs in the optical transmission path in the neighborhood of optical repeater station, a required optical fiber (shown by dashed lines in the figure) may be inserted at the point of the failure for the purpose of repair and the like. At this time, if each of the transmission sections of the upstream and downstream lines has been pumped by the common Raman amplifier, the Raman gain in the transmission section into which the optical fiber has been inserted (at the upstream side in FIG. 15) will differ from the Raman gain in the transmission section into which the optical fiber has not been inserted (at the downstream side in FIG. 15), thereby it becomes difficult to control the Raman gain in the entire optical transmission system. Further, if any measure such as, for example, to reduce the power supplied by the Raman amplifier corresponding to the transmission section into which the optical fiber has been inserted is taken, in order to eliminate an influence on the entire system at the time of the failure and the like as described above, there is caused a problem in that an optical SNR of the transmitted light is degraded.
In addition, in the optical transmission system in which the distributed Raman amplification is performed by pumping each of the transmission sections of the upstream and downstream lines using the common Raman amplifier, there is also caused a problem with regard to a supervisory device. In general, in the optical transmission system, the supervisory device for transferring a supervisory signal indicating transmission conditions of signal light and the like on the system to control an operation of each optical repeater station is provided. The supervisory signal mentioned above is transferred among each optical repeater station, for example, by modulating the pumping light for amplifying the signal light to be superimposed on the signal light. Therefore, in the system configuration in which the Raman amplifier is shared for each transmission section of both the upstream and downstream lines, the supervisory signal indicating particular information can be transferred only in one direction such as only in the upstream direction or only in the downstream direction. In the system in which the upstream line and the downstream line are combined, for example, since supervision is often performed such as by transferring a response signal via the downstream line to the sender side, in response to the supervisory signal transferred via the upstream side from the sender side, it becomes difficult to cope with such supervision.